Sources:
1) https://www.dandwiki.com/wiki/Succubus_(5e_Race)
2) https://forgottenrealms.fandom.com/wiki/Succubus
3) Dnd 5e manuals
(1) Succubi are the iconic depiction of lust. Feared for their typically evil nature and ability to steal one’s soul, succubi are fiends who wield subtle illusions and enchantments to tempt their victims into forfeiting their eternal souls. Succubi inhabit all of the Lower Planes, and the lascivious dark-winged fiends can be found in service to devils, demons, night hags, rakshasas, and yugoloths. Asmodeus, ruler of the Nine Hells, uses these fiends to tempt mortals to perform evil acts. The demon lord Graz’zt keeps succubi and incubi as advisers and consorts.
Succubi often live up to their reputation, acting as seductive females or males to lure males and/or females into relinquishing some of their life-force. Particularly adept at controlling others in the peak of their strength, they’re often employed – albeit reluctantly – by others to act as crowd control, or to tame certain creatures. Most of them share the nature of their reputation; flirtatious. They avoid fighting where possible, instead choosing seduction or fleeing.
The gender-neutral term for Succubi and Incubi is usually either Foocubus or Concubus.
Succubi in their true forms have an appearance similar to that of humans, but are horned, clawed, tailed, winged, and comely. They are typically slender, well toned, and are very rarely overly muscular. Due to their shapechanging abilities, they can tweak their appearances or even change them completely to suit their needs.
(2) In the 15th century DR, circa 1479 DR, they were instead considered to be a form of devil of the Nine Hells, and they tempted mortals into villainy. However, after the Second Sundering of 1487 DR, they were known as simply fiends, found across all the Lower Planes and serving all kinds of evil.
However, succubi often used their shapechanging ability to assume a humanoid form, and could maintain it as a disguise for as long as they liked. Thus mortals rarely saw succubi and incubi in their true forms but they were always attractive to an observer.
(According to this source, there are three types of Succubi: The Demonic Succubi, The Devilish Succubi and the Fiendish Succubi. These three differing types are born within the different Evil Outer Planes of the Great Wheel.)
Demonic succubi lived to seduce and tempt mortal beings of the Prime Material Plane,[3][5][6] and in the end slay them and carry their life-forces back with them, for the destruction of mortals increased the power of the Abyss.
Devilish succubi were adept at winning hearts and breaking hearts, but they also made for cunning and skilled spies and assassins. They served higher-ranked devils as scouts and advisors, as emissaries to key mortals, and of course as concubines. They tempted mortals into sin on behalf of Asmodeus.
Fiendish succubi commonly used their ethereal form to slip through walls to reach a mortal’s bedside, and linger there as they slept. Here, they filled their victim’s dreams with debauched scenes and whispered of forbidden pleasures, tempting them to indulge in dark desires, appetites, and taboos. The more the succubus did this in their dreams, the more vulnerable their victim became to temptation in the waking world.
Summary.
The DnD Lore of the Succubi and Incubi are for more developed then the ‘real’ counterpart. The Succubi seduce men to obtain their life or their seed, or both. The only way for Succubi and Incubi to reproduce is for the Succubi to steal the men’s seed to give to the Incubi to impregnate a (human) female. The Succubi and Incubi have powers and abilities to complete their task.